Tweak kill action #ifdef

Marlin/Marlin_main.cpp

   _delay_ms(250); //Wait to ensure all interrupts routines stopped
   thermalManager.disable_all_heaters(); //turn off heaters again
 
-  #if defined(ACTION_ON_KILL)
+  #ifdef ACTION_ON_KILL
     SERIAL_ECHOLNPGM("//action:" ACTION_ON_KILL);
   #endif